There exist several well known transmit diversity techniques wherein one or several redundancy versions relating to identical data are transmitted on several (at least two) diversity branches “by default” without explicitly requesting (by a feedback channel) further diversity branches (as done in an ARQ scheme by requesting retransmissions). For example the following schemes are considered as transmit diversity:                Site Diversity: The transmitted signal originates from different sites, e.g. different base stations in a cellular environment.        Antenna Diversity: The transmitted signal originates from different antennas, e.g. different antennas of a multi-antenna base station.        Polarization Diversity: The transmitted signal is mapped onto different polarizations.        Frequency Diversity: The transmitted signal is mapped e.g. on different carrier frequencies or on different frequency hopping sequences.        Time Diversity: The transmitted signal is e.g. mapped on different interleaving sequences.        Multicode Diversity: The transmitted signal is mapped on different codes in e.g. a CDMA (Code Division Multiple Access) system.        
There are known several diversity combining techniques. The following three techniques are the most common ones:                Selection Combining: Selecting the diversity branch with the highest SNR for decoding, ignoring the remaining ones.        a Equal Gain Combining: Combining received diversity branches with ignoring the differences in received SNR.        Maximal Ratio Combining: Combining received diversity branches taking the received SNR of each diversity branch into account. The combining can be performed at bit-level (e.g. LLR) or at modulation symbol level.        
Furthermore, a common technique for error detection/correction is based on Automatic Repeat reQuest (ARQ) schemes together with Forward Error Correction (FEC), called hybrid ARQ (HARQ). If an error is detected within a packet by the Cyclic Redundancy Check (CRC), the receiver requests the transmitter to send additional information (retransmission) to improve the probability to correctly decode the erroneous packet.
In WO-02/067491 A1 a method for hybrid ARQ transmissions has been disclosed which averages the bit reliabilities over successively requested retransmissions by means of signal constellation rearrangement.
As shown therein, when employing higher order modulation formats (e.g. M-PSK, M-QAM with log2(M)>2), where more than 2 bits are mapped onto one modulation symbol, the bits have different reliabilities depending on the chosen mapping. This leads for most FEC (e.g. Turbo Codes) schemes to a degraded decoder performance compared to an input of more equally distributed bit reliabilities.
In conventional communication systems the modulation dependent variations in bit reliabilities are not taken into account and, hence, usually the variations remain after combining the diversity branches at the receiver.